The toilets, built by Austrian artist Friedensreich Hundertwasser, have been settling since they were built in 1999, but that subsidence was expected to eventually stop.

Instead, the toilet's guardians say the sinking has accelerated and has now reached an alarming rate.

Ground is slumping, new cracks are appearing, beams are parting from walls, and gaps are opening under columns.

Custodians Christine Kerr and Sharon Davis observe the loos at close quarters every day and are worried by what they've seen in recent weeks.

They point to what had been a hairline crack a month ago, but is now almost wide enough to fit a finger.

A 2cm gap has opened up under a brick column, while a network of fine cracks criss-cross the inside walls.

"It's always been dropping," Ms Kerr said.

"But in the last few weeks it's dropped dramatically. That crack in the ladies' toilet has been there for three years, but it's the rapid acceleration in the past three months that concerns us."

Bay of Islands-Whangaroa Community Board member Johnson Davis said he feared for the toilets' future.

"The last thing I want to see is that those toilets have collapsed or have been shut down, because the council has failed to act."

But the Far North District Council has been monitoring the settlement of the toilets since cracks appeared a number of years ago.

Engineers advised council staff two years ago that the settlement would reduce over time, but it would appear that further settlement has occurred, a council spokesman said.

The council has employed Kerikeri structural engineers Haigh Workman to investigate the problem and suggest options for remedial works.

Haigh Workman has already said that the settlement hasn't damaged the structural integrity of the building and that the toilets are safe for public use.

The council hasn't established the cause of the settling, but a 1.8m diameter stormwater pipe runs across the main street of Kawakawa.

A camera has investigated the stormwater pipes in the area and roots cleared when they have been found.

"There may be other water sources or springs that could destabilise the ground but we have not identified evidence of this," the spokesman said.

"We hope to have a better understanding of the situation when Haigh Workman has completed its investigation."

Ms Kerr, who is also the secretary of the Kawakawa Hundertwasser Park Charitable Trust, said a major leak at the building next door a few years ago had sent huge volumes of water coursing under the toilets.

Repairs could be costly, but there was no way of putting a dollar value on the visitors who came to see the toilets, then stayed on to see the caves and railway, and spent money in cafes and shops.

"The toilets have transformed our town. Perhaps the money isn't going into council coffers but it is going into this area," Ms Kerr said.

Referring to Whangarei's plans for a multimillion-dollar Hundertwasser Arts Centre at the Town Basin, Mr Davis added: "Perhaps if the Whangarei District Council values Hundertwasser, we should relocate the toilets there."